Transmission type optical element and anti-forgery volume hologram

ABSTRACT

A transmission type optical element  10  includes a first hologram  1 ′ having a first surface  1   a  and a second surfaces  1   b  and a second hologram  2 ′ having a third surface  2   a  and a fourth surface  2   b , the first surface  1   a  of the first hologram  1 ′ and the third surface  2   a  of the second hologram  2 ′ being oppositely arranged, in which a first light beam  5 ′ having a first wavelength is made to enter the first hologram  1 ′ from the side of the second surface  1   b , be transmitted through the first hologram  1′ , exit the first hologram  1 ′ from the side of the first surface  1   a , enter the second hologram  2 ′ from the side of the third surface  2   a , be diffracted by the second hologram  2′ , exit the second hologram  2 ′ from the side of the third surface  2   a , enter the first hologram  1 ′ from the side of the first surface  1   a , be diffracted by the first hologram  1′ , exit the first hologram  1 ′ from the side of the first surface  1   a , enter the second hologram  2 ′ from the side of the third surface  2   a , be transmitted through the second hologram  2 ′ and exit the second hologram  2 ′ from the side of the fourth surface  2   b.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims benefit of Japanese Patent Application Nos.2008-206794 and 2008-206795 filed in Japan on Aug. 11, 2008, the entirecontents of which are incorporated herein by this reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a transmission type element havingfirst and second holograms that are Lippmann holograms and havingwavelength selectivity and also to a transmission observation typeanti-forgery volume hologram having wavelength selectivity that isparticularly suitable for forgery prevention.

2. Description of the Related Technology

As general characteristics of holograms, it is known that Lippmannholograms (volume and reflection type holograms) have a high-angleselectivity and high-wavelength selectivity while transmission typeholograms (volume type/surface relief type) give rise to colordispersion to a large extent. FIG. 14 is a schematic illustration of thespectral transmittance distribution of a reflection type hologram andthat of a transmission type hologram. As illustrated in FIG. 14,reflection type holograms have high-wavelength selectivity whereastransmission type holograms have low-wavelength selectivity.

Holography is a technique of recording both the amplitude and the phaseof a light into photosensitive material as interference fringe betweenreference and object light. While ordinary photography can provide onlyan image observed from a single angle of view, a hologram prepared bymeans of this technique can reproduce a three dimensional image obtainedby observing an object from different angles of view. A hologramrequires a sophisticated technique for preparation and also an apparatusthat is sophisticated and costly. Therefore, it is generally difficultto forge or alter a hologram and hence attempts are being made to useholograms as anti-forgery means for certificates and securities,exploiting the difficulty of forgery of holograms.

While a sophisticated optical design technique and costly equipment arerequired to copy a hologram, it is possible to copy a hologram bycontact copy method, which photosensitive material is stacked on masterhologram, and light for recording enters from photosensitive materialside in case of a reflection type hologram and from master hologram sidein case of a transmission type hologram. For example, it has beenproposed to prevent copying a hologram for the purpose of forgery bydividing a hologram into element regions and varying the angle ofreference light for each region (see Patent Document 2: JP-A-08-123305).

An invention relating to an image combiner for using a reflection typehologram in the inside of an element and making light from an imagedisplay means and external light overlap each other has been disclosed(Patent Document 1: JP-A-2004-061731).

However, it is difficult to use the above-described arrangement forsynthesis/separation of light. Light has to enter from the edge of lightguide because of adapting total reflection, so area of incident light islimited by the area of edge of light guide, that results in decrease oftotal amount of light and difficulty in case of applications that needswide area of incident light.

Additionally, when a transmission type hologram is used as an opticalelement, light other than the designed wavelength is also diffractedbecause of low wavelength selectivity, that gives rise to trouble whenused for synthesis/separation of a light path in certain applications.

In view of the above-identified problems of the conventional technology,the object of the present invention is to provide a transmission typeoptical element having excellent wavelength selectivity and atransmission type anti-forgery volume hologram that is not costly andstructurally simple but has excellent wavelength selectivity and isdifficult to be forged.

SUMMARY OF THE INVENTION

According to the present invention, the above object is achieved byproviding a transmission type optical element including: a firsthologram having a first surface and a second surface; and a secondhologram having a third surface and a fourth surface; the first surfaceof the first hologram and the third surface of the second hologram beingoppositely disposed; wherein a first light beam having a firstwavelength is made to enter the first hologram from the side of thesecond surface, be transmitted through the first hologram, exit thefirst hologram from the side of the first surface, enter the secondhologram from the side of the third surface, be diffracted by the secondhologram, exit the second hologram from the side of the third surface,enter the first hologram from the side of the first surface, bediffracted by the first hologram, exit the first hologram from the sideof the first surface, enter the second hologram from the side of thethird surface, be transmitted through the second hologram and exit thesecond hologram from the side of the fourth surface.

Preferably, a transmission type optical element according to the presentinvention as defined above is characterized in that a second light beamhaving a second wavelength different from the first wavelength is madeto enter the first hologram from the side of the second surface, to betransmitted through the first hologram, exit the first hologram from theside of the first surface, enters the second hologram from the side ofthe third surface, to be transmitted through the second hologram andexit the second hologram from the side of the fourth surface.

Preferably, a transmission type optical element according to the presentinvention as defined above is characterized in that the first light beamand the second light beam represent different exit angles to the secondhologram when they represent a same incident angle to the firsthologram.

Preferably, a transmission type optical element according to the presentinvention as defined above is characterized in that the first light beamand the second light beam represent different incident angles to thefirst hologram when they represent a same exit angle to the secondhologram.

In another aspect of the present invention, the above object is achievedby providing a transmission type anti-forgery volume hologram havingwavelength selectivity, including: a first hologram that is a Lippmannhologram having a first surface and a second surface; and a secondhologram that is a Lippmann hologram having a third surface and a fourthsurface; the first surface of the first hologram and the third surfaceof the second hologram being oppositely disposed; wherein a first lightbeam having a first wavelength is made to enter the first hologram fromthe side of the second surface, be transmitted through the firsthologram, exit the first hologram from the side of the first surface,enter the second hologram from the side of the third surface, bediffracted by the second hologram, exit the second hologram from theside of the third surface, enter the first hologram from the side of thefirst surface, be diffracted by the first hologram, exit first hologramfrom the side of the first surface, enter the second hologram from theside of the third surface, be transmitted through the second hologramand exit the second hologram from the side of the fourth surface, that asecond light beam having a second wavelength different from the firstwavelength is made to enter the first hologram from the side of thesecond surface, to be transmitted through the first hologram, exit thefirst hologram from the side of the first surface, enters the secondhologram from the side of the third surface, to be transmitted throughthe second hologram and exit the second hologram from the side of thefourth surface and that the first light beam and the second light beamrepresent different exit angles to the second hologram when theyrepresent a same incidence angle to the first hologram.

Thus, according to the present invention, it is possible to provide acompact transmission type optical element that is not costly andstructurally simple but has excellent wavelength selectivity.

Additionally, according to the present invention, it is possible toprovide a compact anti-forgery volume hologram that is not costly andstructurally simple but has excellent wavelength selectivity and isdifficult to be forged.

Still other objects and advantages of the invention will in part beobvious and will in part be apparent from the specification.

The invention accordingly includes the features of construction,combinations of elements and arrangement of parts which will beexemplified in the construction hereafter set fourth and the scope ofthe invention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a method of preparing a firsthologram 1′;

FIG. 2 is a schematic illustration of a method of reproducing a firsthologram 1′;

FIG. 3 is a schematic illustration of a method of preparing a secondhologram 2′;

FIG. 4 is a schematic illustration of the configuration of Example 1 oftransmission type optical element that is prepared and provided withwavelength selectivity;

FIG. 5 is a schematic illustration of Example 1 of synthesis of light bya transmission type optical element;

FIG. 6 is a schematic illustration of Example 1 of separation of lightby a transmission type optical element;

FIG. 7 is a schematic illustration of the configuration of Example 2 oftransmission type optical element that is prepared and provided withwavelength selectivity;

FIG. 8 is a schematic illustration of Example 2 of synthesis of light bya transmission type optical element;

FIG. 9 is a schematic illustration of Example 2 of separation of lightby a transmission type optical element;

FIGS. 10A to 10C are schematic illustrations of a design method of atransmission type optical element or an anti-forgery volume hologram;

FIG. 11 is a schematic illustration of the incidence angle dependency ofthe diffraction efficiency when the designed incident angle is madeequal to 0°;

FIG. 12 is a schematic illustration of light synthesis element 20 usinga transmission type optical element 10 of an embodiment of the presentinvention;

FIG. 13 is a schematic illustration of an attempt of forgery of ananti-forgery volume hologram;

FIG. 14 is a schematic illustration of the spectral transmittancedistribution of a reflection type hologram and that of a transmissiontype hologram; and

FIG. 15 is a schematic illustration of a known cross dichroic prism 50.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Now, an embodiment of transmission type optical element according to thepresent invention that has excellent wavelength selectivity and also anembodiment of volume hologram according to the present invention that isnot costly and structurally simple but has excellent wavelengthselectivity and is difficult to be forged will be described below byreferring to the accompanying drawings.

A transmission type optical element 10 or an anti-forgery volumehologram 10 according to the present invention that is provided withwavelength selectivity includes two Lippmann holograms that areoppositely arranged. A first hologram 1′ that is prepared in a manner asillustrated in FIG. 1 and a second hologram 2′ that will be describedhereinafter are employed as Lippmann holograms.

To prepare a transmission type optical element 10 or an anti-forgeryvolume hologram 10 of this embodiment, firstly a first hologram that isa prerequisite is prepared. FIG. 1 is a schematic illustration of amethod of preparing a first hologram 1′.

Firstly, as illustrated in FIG. 1, a first object light beam 3 having afirst wavelength is made to enter a first photosensitive material 1 forrecording hologram 1′, which may be a photopolymer or a silver halidematerial, at a predetermined incident angle from the side of a firstsurface 1 a. At the same time, a first reference light beam 4 having afirst wavelength that is emitted from a light source same as that of thefirst object light beam 3 and collimated or substantially collimated ismade to enter a photosensitive material 1 at a predetermined incidenceangle that is different from the incidence angle of the first objectlight beam 3, from the side of a second surface 1 b that is opposite tothe incident side of the first object light beam 3.

As the first object light beam 3 and the first reference light beam 4are made to enter the photosensitive material 1, the first object lightbeam 3 and the first reference light beam 4 interfere with each other.Thereafter, the photosensitive material 1 is subjected to apost-treatment to prepare the first hologram 1′.

In an example above, photopolymer was used for green color as the firsthologram recording photosensitive material 1 for preparing a Lippmannhologram. A DPSS laser having a wavelength of 532 nm was used for theimaging.

FIG. 2 is a schematic illustration of a step of reproducing the Lippmannhologram. As a first reproduction illumination light beam 5 thatincludes the first wavelength used at the time of recording and proceedsin a direction opposite to the first reference light beam 4 illustratedin FIG. 1 is made to enter the prepared first hologram 1′ from the sideof the first surface 1 a, the first reproduction light beam 6 thatproceeds in a direction opposite to the first object light beam 3illustrated in FIG. 1 is diffracted and exits from the side of the firstsurface 1 a.

FIG. 3 is a schematic illustration of a step of preparing the Lippmanntype second hologram 2′. A second object light beam 3′ having the firstwavelength that represents an incidence angle substantially equal to theincident angle of the first reference light beam 4 of the first hologram1′ and is substantially collimated and a second reference light beam 4′having the first wavelength same as the wavelength used when recordingthe first hologram 1′ that is emitted from a light source same thesecond object light beam 3′ and substantially collimated are made toenter the photosensitive material 2 for recording second hologram 2′respectively from the side of the third surface 2 a and from the side ofthe fourth surface 2 b that is opposite to the entering side of thesecond object light beam 3′ and interfere with each other and then thephotosensitive material 2 is subjected to a post-treatment to preparethe second hologram 2′.

FIG. 4 is a schematic illustration of the configuration of Example 1 oftransmission type optical element 10 or anti-forgery volume hologram 10that is prepared and provided with wavelength selectivity, illustratingthe configuration thereof. The first surface 1 a of the first hologram1′ and the third surface 2 a of the second hologram 2′ are oppositelyarranged in the transmission type optical element 10 or the anti-forgeryvolume hologram 10.

As the second reproduction illumination light beam 5′ having the firstwavelength is made to enter the first hologram 1′ from the side of thesecond surface 1 b, the second reproduction illumination light beam 5′is transmitted through the first hologram 1′ and diffracted in thesecond hologram 2′ to become the second reproduction light beam 6′.Since the exit angle of the second reproduction light beam 6′ isdesigned to be substantially equal to the incidence angle of thereproduction illumination light beam 5 of the first hologram 1′, it isdiffracted in the first hologram 1′ to become the first reproductionlight beam 6. Then, the first reproduction light beam 6 is transmittedthrough the second hologram 2′ to go out. While the incidence angle ofthe first hologram 1′ and the exit angle of the second hologram 2′ areequal to each other, they may be displaced from each other depending onthe hologram material.

As for the diffracting function of the element, the element has afunction of operating as a transmission type hologram that diffracts thesecond reproduction illumination light beam 5′ to the direction of thefirst reproduction light beam 6 and also the wavelength selectivity of aLippmann hologram.

Now, synthesis of light of this embodiment will be described below. FIG.5 is a schematic illustration of Example 1 of synthesis of light by thetransmission type optical element 10.

When a light beam 11 having the second wavelength that is different fromthe first wavelength is made to enter the first hologram 1′ from theside of the second surface 1 b at an angle substantially equal to theincidence angle of the first reproduction light beam 6, the light beam11 of the second wavelength exits at an angle substantially equal toexit angle of the first reproduction light beam 6.

Therefore, as illustrated in FIG. 5, when the second reproductionillumination light beam 5′ having the first wavelength is made to enterthe first hologram 1′ from the side of the second surface 1 b and thelight beam 11 having the second wavelength that is different from thefirst wavelength is made to enter the first hologram 1′ from the side ofthe second surface 1 b at an angle substantially equal to the incidentangle of the first reproduction light beam 6, the second reproductionillumination light beam 5′ and the light beam 11 of the secondwavelength are synthetically combined and exit at an angle substantiallyequal to the exit angle of the first reproduction light beam 6.

As for the synthesis of light, when hologram 1′ was recorded at 532 nm,a light beam of a wavelength of 647.1 nm emitted from a Kr laser so asto enter the first hologram 1′ at an angle substantially equal to theincidence angle of the first reproduction light beam 6 was transmittedwithout being diffracted to make sure that it can be syntheticallycombined with a diffracted light beam of 532 nm.

Now, separation of light of this embodiment will be described below.FIG. 6 is a schematic illustration of Example 1 of separation of lightby the transmission type optical element 10.

When a light beam 11 of the second wavelength that is different from thefirst wavelength is made to enter the first hologram 1′ from the side ofthe second surface 1 b at an angle substantially equal to the incidentangle of the second reproduction illumination light beam 5′, the lightbeam 11 of the second wavelength is made to exit at an angle same as theexit angle of the light beam of the 0th order of the reproductionillumination light beam 5′ and different from the exit angle of thereproduction light beam 6.

Therefore, as illustrated in FIG. 6, when the second reproductionillumination light beam 5′ having the first wavelength is made to enterthe first hologram 1′ from the side of the second surface 1 b and thelight beam 11 having the second wavelength that is different from thefirst wavelength is made to enter the first hologram 1′ from the side ofthe second surface 1 b at an angle substantially equal to the incidentangle of the second reproduction illumination light beam 5′, the secondreproduction illumination light beam 5′ and the light beam 11 of thesecond wavelength are separated from each other and the secondreproduction illumination light beam 5′ having the first wavelength ismade to exit at an angle substantially equal to the exit angle of thefirst reproduction light beam 6, while the light beam 11 of the secondwavelength is made to exit at an angle substantially equal to the exitangle of the light beam of the 0th order of the second reproductionillumination light beam 5′.

As for separation of light, when hologram 1′ was recorded at 532 nm, alight beam of a wavelength of 647.1 nm emitted from a Kr laser so as toenter the first hologram 1′ at an angle substantially equal to theincidence angle of the second reproduction illumination light beam 5′was transmitted without being diffracted to make sure that it can beseparated from a diffracted light beam of 532 nm.

A reproduction light beam may be made to enter the transmission typeoptical element 10 in an opposite direction. FIG. 7 is a schematicillustration of the configuration of Embodiment 2 of transmission typeoptical element 10 that is prepared and provided with wavelengthselectivity, illustrating the configuration thereof.

As a third reproduction illumination light beam 7 that proceeds in adirection opposite to the proceeding direction of the first reproductionlight beam 6 is made to enter the second hologram 2′ from the side ofthe fourth surface 2 b, the third reproduction illumination light beam 7is transmitted through the second hologram 2′ and diffracted by thefirst hologram 1′ to become the third reproduction light beam 7′. Sincethe third reproduction light beam 7′ proceeds in a direction opposite tothe proceeding direction of the reproduction light beam 6′ of the secondhologram 2′, it is diffracted again in the second hologram 2′ to becomea fourth reproduction light beam 8. Then, the fourth reproduction lightbeam 8 is transmitted through the first hologram 1′ and exits in adirection opposite to the direction of the second reproductionillumination light beam 5′.

Thus, as for the diffracting function of the element, the element has afunction of operating as a transmission type hologram that diffracts thethird reproduction illumination light beam 7 in the direction of thefourth reproduction light beam 8 and also the wavelength selectivity ofa Lippmann hologram.

Now, synthesis of light of this embodiment will be described below. FIG.8 is a schematic illustration of Example 2 of synthesis of light by thetransmission type optical element 10.

When a light beam having the second wavelength that is different fromthe first wavelength is made to enter the second hologram 2′ from theside of the fourth surface 2 b at an angle substantially equal to theincident angle of the fourth reproduction light beam 8, the light beam11 of the second wavelength exits at an angle substantially equal toexit angle of the fourth reproduction light beam 8.

Therefore, as illustrated in FIG. 8, when the third reproductionillumination light beam 7 having the first wavelength is made to enterthe second hologram 2′ from the side of the second surface 2 b and thelight beam 11 having the second wavelength that is different from thefirst wavelength is made to enter the second hologram 2′ from the sideof the fourth surface 2 b at an angle substantially equal to theincident angle of the fourth reproduction light beam 8, the thirdreproduction light beam 7 and the light beam 11 of the second wavelengthare synthetically combined and exit at an angle substantially equal tothe exit angle of the fourth reproduction light beam 8.

Now, separation of light of this embodiment will be described below.FIG. 9 is a schematic illustration of Example 2 of separation of lightby the transmission type optical element 10.

When a light beam 11 of the second wavelength that is different from thefirst wavelength is made to enter the second hologram 2′ from the sideof the fourth surface 2 b at an angle substantially equal to theincident angle of the third reproduction illumination light beam 7, thelight beam 11 of the second wavelength is made to exit at an angle sameas the exit angle of the light beam of the 0th order of the thirdreproduction illumination light beam 7 and different from the exit angleof the fourth reproduction light beam 8.

Therefore, as illustrated in FIG. 9, when the third reproductionillumination light beam 7 having the first wavelength is made to enterthe second hologram 2′ from the side of the fourth surface 2 b and thelight beam 11 having the second wavelength that is different from thefirst wavelength is made to enter the second hologram 2′ from the sideof the fourth surface 2 b at an angle substantially equal to theincident angle of the third reproduction illumination light beam 7, thethird reproduction illumination light beam 7 and the light beam 11 ofthe second wavelength are separated from each other and the thirdreproduction illumination light beam 7 having the first wavelength ismade to exit at an angle substantially equal to the exit angle of thefourth reproduction light beam 8, while the light beam 11 of the secondwavelength is made to exit at an angle substantially equal to the exitangle of the light beam of the 0th order of the third reproduction lightbeam 7.

Now, the method of designing the transmission type optical element 10 orthe anti-forgery volume hologram 10 of this embodiment will be describedbelow. FIG. 10 is a schematic illustration of the design method of thetransmission type optical element 10 or the anti-forgery volume hologram10.

Firstly, as illustrated in FIG. 10A, the desired ultimate exit angle ofthis embodiment is determined. Then, as illustrated in FIG. 10B, anincident angle that does not allow the first hologram 1′, which is theproximal hologram as viewed from the entering side, to diffract isselected.

FIG. 11 is a schematic illustration of the incident angle dependency ofthe diffraction efficiency when the designed incident angle is madeequal to 0°. As illustrated in FIG. 11, since the first hologram 1′diffracts within a range of ±10° of the designed incident angle, anyincident angle that is within the range of ±10° of the desired exitangle of the transmission type optical element 10 is desirably avoidedfor the incident angle to the first hologram 1′.

Then, as illustrated in FIG. 10C, a reflective angle at which the secondhologram 2′, which is the distal hologram as viewed from the enteringside, does not diffract any outgoing light beam is selected. In thiscase again, as illustrated in FIG. 11, since the second hologram 2′diffracts within a range of ±10° of the designed incidence angle, anyreflective angle that is within the range of ±10° of the desired exitangle of the transmission type optical element 10 is desirably avoidedwhen selecting the reflective angle of the second hologram 2′.

The transmission type optical element 10 that is prepared in this way iscompact, not costly and structurally simple but has excellent wavelengthselectivity.

Furthermore, the anti-forgery volume hologram 10 prepared in this wayprovides effects same as those of the transmission type hologram.Additionally, it is compact, not costly and structurally simple but hasexcellent wavelength selectivity and an excellent anti-forgery effect.

Now, a light synthesis element formed by using such a transmission typeoptical element 10 will be described below.

FIG. 15 is a schematic illustration of a known cross dichroic prism 50.As illustrated in FIG. 15, a cross dichroic prism 50 to be used forcolor synthesis and other applications is an element for synthesizinglight elements having wavelengths of RGB. Known cross dichroic prismsrequire a high degree of angular precision for the rectangular prismsurfaces of each prism and hence are costly.

FIG. 12 is a schematic illustration of a light synthesis element 20formed by using this embodiment of a transmission type optical element10 of the present invention. As illustrated in FIG. 12, the lightsynthesis element 20 is an element formed by bonding a transmission typeoptical element 10 of this embodiment and a dichroic mirror 15 for R.

Firstly, the operation of the light synthesis element 20 will bedescribed below. Firstly, the transmission type optical element 10 ofthis embodiment operates as transmission type optical element for B andthe element synthetically combines a light beam having the wavelength ofB and a light beam having the wavelength of G. Subsequently, the lightbeam produced by the transmission type optical element 10 as a result ofthe synthetic combination is then further synthetically combined with alight beam having the wavelength of R by the dichroic mirror 15 for R.Thus, the light synthesis element 20 can synthetically combine lightbeams respectively having the wavelengths of R, G and B.

In this way, a light synthesis element 20 formed by using a transmissiontype optical element 10 of this embodiment can be prepared in a simplemanner by bonding the transmission type optical element 10 and adichroic mirror 15 for R so that it is not costly but lightweight. Notethat the wavelengths that are used by the transmission type opticalelement 10 for synthetic combination are not limited to those of G and Band also the dichroic mirror 15 is not limited to a dichroic mirror forR. In other words, some other element and/or some other dichroic mirrormay alternatively be used for the purpose of the present invention.

Next, an attempt of forgery of the anti-forgery volume hologram 10 willbe described below.

FIG. 13 is a schematic illustration of an attempt of forgery of theabove-described anti-forgery volume hologram 10. Referring to FIG. 13, aforged hologram was produced by arranging a photosensitive material 30for copying at the side of the second hologram 2′ of the anti-forgeryvolume hologram 10 that is opposite to the first hologram 1′ andirradiating a reproduction illumination light beam 5′ from the side ofthe first hologram 1′. Interference fringes were produced by thereproduction light beam 6 and the transmitted light beam 8 and recordedin the photosensitive material 30 for forgery.

Thus, man can recognize at a glance the difference between the forgedhologram produced in this way that is a transmission type hologramrepresenting color dispersion to a large extent like holograms of theconventional technology and the anti-forgery volume hologram 10 having ahigh degree of wavelength selectivity.

In the case of a hologram according to the present invention that isrecorded with a wavelength of 532 nm, for example, a transmitted andreproduced image of green (at or near 532 nm) can be observed within arange of about ±10° of the designed incident angle. In the case of anordinary transmission type hologram, a transmitted and reproduced imagecan be observed within a broader range of incident angle because of awide color dispersion and reproduced image of the rainbow colors isobserved.

While the present invention is described above by way of embodiments oftransmission type optical element 10 or anti-forgery volume hologram 10,the present invention is by no means limited by those embodiments, whichmay be modified in various different ways. For instance, the firsthologram 1′ or the second hologram 2′ may be provided with an adhesivelayer and bonded to other materials by means of the adhesive layer.

When an anti-forgery volume hologram according to the present inventionis employed for a label, the label may typically be made to have aprotection layer/a hologram layer/an adhesive layer and bonded to adesired base material by means of the adhesive layer. When it isemployed for a transfer foil, the transfer foil may typically be made tohave a protection layer/a hologram layer/a HS layer and bonded to adesired base material by means of the HS layer.

Thus, according to the present invention, it is possible to provide acompact transmission type optical element that is not costly andstructurally simple but has excellent wavelength selectivity or acompact anti-forgery volume hologram that is not costly and structurallysimple but has excellent wavelength selectivity and is difficult to beforged.

1. A transmission type optical element comprising: a first hologramhaving a first surface and a second surfaces; and a second hologramhaving a third surface and a fourth surface; the first surface of thefirst hologram and the third surface of the second hologram beingoppositely arranged; wherein a first light beam having a firstwavelength is made to enter the first hologram from the side of thesecond surface, be transmitted through the first hologram, exit thefirst hologram from the side of the first surface, enter the secondhologram from the side of the third surface, be diffracted by the secondhologram, exit the second hologram from the side of the third surface,enter the first hologram from the side of the first surface, bediffracted by the first hologram, exit the first hologram from the sideof the first surface, and enter the second hologram from the side of thethird surface, be transmitted through the second hologram and exit thesecond hologram from the side of the fourth surface.
 2. The transmissiontype optical element according to claim 1, wherein a second light beamhaving a second wavelength different from the first wavelength is madeto enter the first hologram from the side of the second surface, to betransmitted through the first hologram, exit the first hologram from theside of the first surface, and enters the second hologram from the sideof the third surface, to be transmitted through the second hologram andexit the second hologram from the side of the fourth surface.
 3. Thetransmission type optical element according to claim 2, wherein thefirst light beam and the second light beam represent exit angles thatare different from each other to the second hologram when they representa same incidence angle to the first hologram.
 4. The transmission typeoptical element according to claim 2, wherein the first light beam andthe second light beam represent incidence angles that are different fromeach other to the first hologram when they represent a same exit angleto the second hologram.
 5. A transmission type anti-forgery volumehologram having wavelength selectivity, comprising: a first hologramthat is a Lippmann hologram having a first surface and a second surface;and a second hologram that is a Lippmann hologram having a third surfaceand a fourth surface; the first surface of the first hologram and thethird surface of the second hologram being oppositely arranged; whereina first light beam having a first wavelength is made to enter the firsthologram from the side of the second surface, be transmitted through thefirst hologram, exit the first hologram from the side of the firstsurface, enter the second hologram from the side of the third surface,be diffracted by the second hologram, exit the second hologram from theside of the third surface, enter the first hologram from the side of thefirst surface, be diffracted by the first hologram, exit the firsthologram from the side of the first surface, enter the second hologramfrom the side of the third surface, be transmitted through the secondhologram and exit the second hologram from the side of the fourthsurface, that a second light beam having a second wavelength differentfrom the first wavelength is made to enter the first hologram from theside of the second surface, to be transmitted through the firsthologram, exit the first hologram from the side of the first surface,enters the second hologram from the side of the third surface, to betransmitted through the second hologram and exit the second hologramfrom the side of the fourth surface and that the first light beam andthe second light beam represent exit angles that are different from eachother relative to the second hologram when they represent a sameincidence angle to the first hologram.